In general, the invention relates to the field of radar systems. More particularly, the invention relates to the field of ground penetrating radar systems.
There are numerous applications in which it is useful to view images of underground objects or objects embedded in the ground or in a material such as concrete, that would not otherwise be visible. For example, it is helpful to utility workers to see images of pipes, cables and conduits that are underground where they are about to dig a hole or a trench. It would be even more helpful if the workers could see a three-dimensional image, so that if an imaged object is deep in the ground below the depth they intend to work, they need not be concerned with the object.
Similarly, a system that would allow workers to see images in three dimensions beneath the surface of the ground would be useful in the field of mine detection. This field has become increasingly important as populations have been moving back into previously war-torn areas of the earth. Frequently, when a war ends, civilians move back into a mined area before the authorities can mount a demining operation. Many civilians are injured or killed by such mines even after a war is over. During a war, demining operations help clear a mine field before foot soldiers or vehicles cross it.
In such demining operations, it is useful to know accurately not only the surface coordinates of a buried mine, but also the depth at which the mine is buried. It is also important that the equipment used for demining is mobile and that it is capable of exploring a large amount of territory in a short period of time, while maintaining an accurate and detailed record of the objects detected. Such information allows personnel to make intelligent decisions regarding the methods and equipment that are necessary to remove the detected mines.
The invention is a ground penetrating radar system mounted on a cart to achieve the desired mobility. The system uses two offset banks of interleaved transmit and receive antennas to achieve the desired accuracy. The receive and transmit antennas are properly oriented with respect to each other to reduce cross coupling and maximize desired subsurface echoes. The system uses nearfield beam forming, which is accomplished through fully coherent signal processing and synthetic aperture reception and processing, to image buried objects in three dimensions. The system displays a plan, or top, view and a side view of the area being scanned to provide a three dimensional perspective on a two dimensional computer screen.
In general, in one aspect, the invention features a ground penetrating radar system which includes a cart configured to be movable along the ground. A computer is mechanically coupled to the cart. A radar electronics module is mechanically coupled to the cart and electrically coupled to the computer. A first antenna array is mechanically coupled to the cart, electrically coupled to the radar electronics module, and oriented to radiate into the ground and receive radiation from the ground. A second antenna array is mechanically coupled to the cart, electrically coupled to the radar electronics module, and oriented to radiate into the ground and receive radiation from the ground. A movement detector, which is configured to detect movement of the cart, is coupled to the computer. The computer is configured to trigger the radar electronics module when the computer detects that the cart has moved a predefined distance.
Implementations of the invention may include one or more of the following. The radar electronics module may include a first radar electronics module electrically coupled to the first antenna array and a second radar electronics module electrically coupled to the second antenna array. The first antenna array may be configured to radiate and receive radiation from a first series of points along a first set of curves parallel to the direction of movement of the cart. The second antenna array may be configured to radiate and receive radiation from a second series of points along a second set of curves parallel to the direction of movement of the cart. The first set of curves may be interleaved with the second set of curves.
In general, in another aspect, the invention features a ground penetrating radar system including a first bank of receive antennas arranged along a first axis, a first bank of transmit antennas arranged along a second axis substantially parallel to the first axis and horizontally displaced from the first axis, a second bank of receive antennas arranged along a third axis substantially parallel to the first axis and horizontally displaced from the first axis, and a second bank of transmit antennas arranged along a fourth axis substantially parallel to the first axis and horizontally displaced from the first axis. A first radar electronics module is coupled to the first bank of transmit antennas and the first bank of receive antennas. A second radar electronics module is coupled to the second bank of transmit antennas and the second bank of receive antennas. The transmit antennas in the first bank of transmit antennas are interleaved with the receive antennas in the first bank of receive antennas and the transmit antennas in the second bank of transmit antennas are interleaved with the receive antennas in the second bank of receive antennas. The receive antennas in the first bank of transmit antennas are offset along the first axis from the receive antennas in the second bank of transmit antennas.
Implementations of the invention may include one or more of the following. The first bank of transmit antennas may be offset along the second axis with respect to the second bank of transmit antennas. The banks of receive antennas may alternate with the banks of transmit antennas. Each transmit antenna may be adjacent to at least one receive antenna. Each transmit antenna may be oriented to minimize electromagnetic coupling to at least one of its adjacent receive antennas. Each transmit antenna may include at least one spiral arm of conductive material. Each receive antenna may include at least one spiral arm of conductive material. A tangent to the inside of the spiral arm at the edge of a transmit antenna may be substantially perpendicular to a tangent to the inside of the spiral arm at the edge of a receive antenna adjacent to the transmit antenna. Each transmit antenna may include two spiral arms of conductive material. Each receive antenna may include two spiral arms of conductive material.
The transmit antennas and the receive antennas may have faces with centers. Two adjacent first bank receive antennas from the first bank of receive antennas and a first bank transmit antenna from the first bank of transmit antennas interleaved between the two adjacent first bank receive antennas may be positioned such that lines between the centers of the faces of the two adjacent first bank receive antennas and the interleaved first bank transmit antenna form a first triangle having sides of approximately the same length. Two adjacent second bank receive antennas from the second bank of receive antennas and a second bank transmit antenna from the second bank of transmit antennas interleaved between the two adjacent second bank receive antennas may be positioned such that lines between the centers of the faces of the two adjacent second bank receive antennas and the interleaved second bank transmit antenna form a second triangle having sides of approximately the same length. A vertex of the first triangle may be displaced in the direction of the first axis relative to a corresponding vertex of the second triangle by an amount substantially equal to one-half the distance from the center of one side of the first triangle to the center of another side of the first triangle.
The third axis may be horizontally displaced from the first axis by an amount substantially equal to eight times the distance from the center of one side of the first triangle to the center of another side of the first triangle. The transmit antennas may not be required to be in contact with the ground when in operation. The receive antennas may not be required to be in contact with the ground when in operation.
In general, in another aspect, the invention features a ground penetrating radar system including a digital module. The digital module includes a direct digital synthesizer configured to generate a digital IF reference signal. A digital to analog converter is coupled to the direct digital synthesizer and is configured to convert the digital IF reference signal to an analog IF transmit signal. An analog to digital converter is configured to convert an analog IF receive signal to a digital IF receive signal. A digital down converter is configured to digitally mix the digital IF receive signal with the digital IF reference signal to produce an in-phase product and the digital IF reference signal shifted in phase by ninety degrees to produce a quadrature product. The ground penetrating radar system includes an RF module coupled to the digital module. The RF module includes an up-converter configured to convert the analog IF transmit signal into a transmit signal and a down-converter configured to convert a receive signal into an analog IF receive signal. The system includes a transmit antenna array coupled to the up-converter for radiating the transmit signal and a receive antenna array coupled to the down-converter for receiving the receive signal.
Implementations of the invention may include one or more of the following. The transmit antenna array may include a plurality of transmit antennas. The receive antenna array may include a plurality of receive antennas. The system may include a digital signal processor. The system may include a transmit switch for applying the transmit signal to one of the plurality of transmit antennas. The transmit switch may be controlled by the digital signal processor. The system may include a receiver switch for receiving the receive signal from one of the plurality of receive antennas. The receiver switch may be controlled by the digital signal processor. The digital signal processor may control the direct digital synthesizer, the digital down converter, the up-converter and the down-converter. The transmit signal may be a stepped-frequency transmit signal. The receive signal may be a stepped-frequency receive signal. The system may include a computer coupled to a processor through an extensible network. The processor may be configured to communicate with the digital signal processor. The extensible network may be a local area network, e.g., ETHERNET network.
In general, in another aspect, the invention features a ground penetrating radar system including a digital module configured to generate an analog IF transmit signal and to receive an analog IF receive signal. The system includes an RF module, which includes a triple-heterodyne up-converter for converting an analog IF transmit signal into a stepped-frequency transmit signal. The RF module also includes a triple-heterodyne frequency converter for converting a stepped-frequency receive signal into an analog IF receive signal. The system includes a transmit antenna coupled to the up-converter for radiating the stepped-frequency transmit signal and a receive antenna coupled to the down-converter for receiving the stepped-frequency receive signal.
Implementations of the invention may include one or more of the following. The triple-heterodyne up-converter may include a first up-converter configured to mix the analog IF transmit signal with a signal from a first local oscillator to produce a first intermediate signal and an aliased first intermediate signal. The triple-heterodyne up-converter may include a first filter coupled to the first up-converter for substantially rejecting the aliased first intermediate signal. The triple-heterodyne up-converter may include a second up-converter coupled to the first filter configured to mix the first intermediate signal with a signal from a second local oscillator to produce a second intermediate signal and an aliased second intermediate signal. The triple-heterodyne up-converter may include a second filter coupled to the second up-converter for substantially rejecting the aliased second intermediate signal. The triple-heterodyne up-converter may include a down-converter coupled to the second filter configured to mix the second intermediate signal with a stepped frequency signal to produce the stepped-frequency transmit signal and an aliased stepped-frequency transmit signal. The stepped-frequency transmit signal may have substantially no frequency components in the pass bands of the first filter or the second filter. The triple-heterodyne up-converter may include a third filter coupled to the down-converter for substantially rejecting the aliased stepped-frequency transmit signal.
The triple-heterodyne up converter may include an up-converter configured to mix the stepped-frequency receive signal with a stepped-frequency signal to produce a first intermediate signal and an aliased first intermediate signal. The triple-heterodyne up-converter may include a first filter coupled to the first up-converter for substantially rejecting the aliased first intermediate signal. The triple-heterodyne up-converter may include a first down-converter coupled to the first filter configured to mix the first intermediate signal with a signal from a first local oscillator to produce a second intermediate signal and an aliased second intermediate signal. The triple-heterodyne up-converter may include a second filter coupled to the first down-converter for substantially rejecting the aliased second intermediate signal. The triple-heterodyne up-converter may include a second down-converter coupled to the second filter configured to mix the second intermediate signal with a second local oscillator to produce the analog IF receive signal and an aliased analog IF receive signal. The triple-heterodyne up-converter may include a third filter coupled to the second down-converter for substantially rejecting the aliased analog IF receive signal.
In general, in another aspect, the invention features a ground penetrating radar system including a transmitter, a receiver, an array of transmit antennas, an array of receive antennas interleaved with the array of transmit antennas, a transmit switch configured to selectively couple the transmitter to one of the array of transmit antennas and a receive switch configured to selectively couple the receiver to one of the array of receive antennas. The array of transmit antennas is arranged in one or more rows. The array of receive antennas is arranged in one or more rows. Each row is parallel to, adjacent to and offset from one of the rows of transmit antennas, so that each receive antenna in a row except one is adjacent to two transmit antennas, and each transmit antenna in a row except one is adjacent to two receive antennas. The transmit switch and the receive switch are configured to couple the transmitter and receiver, respectively, to a first transmit antenna and a first adjacent receive antenna, and subsequently to the first transmit antenna and a second adjacent receive antenna.
In general, in another aspect, the invention features a method for collecting and displaying data from a ground penetrating radar system, which includes a plurality of transmit antennas and a plurality of receive antennas. Each transmit antenna, except one, has two adjacent receive antennas. The system is mounted on a movable cart. The method includes collecting raw data. Collecting raw data includes (a) selecting a first of the plurality of transmit antennas. Collecting raw data further includes (b) selecting a first receive antenna that is adjacent to the selected transmit antenna. Collecting raw data further includes (c) collecting data using the selected transmit antenna and the selected receive antenna to produce raw data. The raw data collected at spatial location (xm, yn) is denoted by {tilde over (xcexa8)}mnp where the indices m, n are used to denote position in a grid of spatial locations where data has been collected, and p is an index ranging from 1 to P corresponding to the frequency fp at which the data was collected. Collecting raw data includes (d) repeating step (c) for both receive antennas adjacent to the selected transmit antenna. Collecting raw data further includes (e) repeating steps b, c and d for all transmit antennas. Collecting raw data further includes repeating steps a, b, c, d, and e each time the cart moves to a new location. The method further includes preconditioning the raw data to produce preconditioned data, analyzing the preconditioned data, and displaying images of the analyzed data.
Implementations of the invention may include one or more of the following. Preconditioning the raw data to produce preconditioned data may include (g) removing a constant frequency component and a system travel time delay, (h) removing a transmit-antenna to receive-antenna coupling effect, (i) prewhitening, and (j) repeating steps (g), (h) and (i) for each spatial location of the raw data.
Removing a constant frequency component and a system travel time delay may include applying the following equation:             Ψ      ^        mnp    =            (                                    Ψ            ~                    mnp                -                              1            P                    ⁢                                    ∑                              p                =                1                            P                        ⁢                          xe2x80x83                        ⁢                                          Ψ                ~                            mnp                                          )        ⁢                  exp        ⁢                  (                                    ⅈ              ·              2                        ⁢                          π              ·                              f                p                            ·              τ                                )                    .      
Removing the transmit-antenna to receive-antenna coupling effect may include applying the following equation:
xcexa8mnp={circumflex over (xcexa8)}mnpxe2x88x92{circumflex over (xcexa8)}mxc3x1p
where {circumflex over (xcexa8)}mxc3x1p is an in track reference scan.
Removing the transmit-antenna to receive-antenna coupling effect may include applying the following equation:       Ψ    mnp    =                    Ψ        ^            mnp        -                  1                              N            2                    -                      N            1                    +          1                    ·                        ∑                      n            =                          N              1                                            N            2                          ⁢                  xe2x80x83                ⁢                              Ψ            ^                    mnp                    
where N1 and N2 define a region to be imaged.
Removing the transmit-antenna to receive-antenna coupling effect may include applying the following equation:       Ψ    mnp    =            ∑              q        =                  -          Q                    Q        ⁢          xe2x80x83        ⁢                  a        q            ·                        Ψ          ^                          m          ,                      n            +            q                    ,          p                    
where aq are digital filter coefficients chosen to reject low frequency spatial energy.
Removing the transmit-antenna to receive-antenna coupling effect may include applying the following equation:
xcexa8mnp={circumflex over (xcexa8)}mnpxe2x88x92{circumflex over (xcexa8)}{tilde over (m)}np
where {circumflex over (xcexa8)}{tilde over (m)}np is a cross line reference scan.
Prewhitening may include applying the following equation:
xcex3mnp=bpxc2x7xcexa8mnp
where bp are frequency dependent weights.
Analyzing the preconditioned data may include applying the following equation:       I    mnp    =            1                        (                                    2              ⁢              U                        +            1                    )                ⁢                  (                                    2              ⁢              V                        +            1                    )                ⁢        P              ·                  ∑                  u          =                      -            U                          U            ⁢              xe2x80x83            ⁢                        ∑                      v            =                          -              V                                V                ⁢                  xe2x80x83                ⁢                              ∑                          p              =                              P                1                                                    P              2                                ⁢                      xe2x80x83                    ⁢                                    γ                                                m                  +                  u                                ,                                  n                  +                  v                                ,                p                                      ⁢                          exp              ⁢                              (                                                      ⅈ                    ·                    2                                    ⁢                                      π                    ·                                          f                      p                                        ·                                          τ                      uvw                                                                      )                                                        
where
Imnw is the complex image value at spatial location (xF,m, yF,n, zw);
U is the SAR array size in the cross-track direction;
V is SAR array size in the along track direction;
(fP1, fP2) is the frequency processing band;
xcfx84uvw is the travel time from source (u, v) in the SAR array down to a focal point at depth zw and back up to receiver (u, v) in the SAR array;             x              F        ,        m              =                            3          ⁢          d                4            +                                    (                          m              -              1                        )                    ⁢          d                4              ;
yF,m=0.933013d+(nxe2x88x921)dy;
d=5.52 inches; and
dy=scan spacing.
The transmit antennas and the receive antennas may be in contact with the ground and the following equation may apply:       τ    uvw    =            1              c        g              ⁡          [                                    (                                          x                                  s                  ,                  u                                            -                              x                                  r                  ,                  u                                                      )                    2                +                              (                                          y                                  s                  ,                  v                                            -                              y                                  r                  ,                  v                                                      )                    2                +                  z          w          2                    ]      
where (xs,u, ys,v) and (xr,u, yr,v) are the location of the transmit and receive antennas and cg is the speed of light in the ground.
Displaying images of the analyzed data may include computing a plan view image of the analyzed data, computing a side view image of the analyzed data, and displaying the plan view image and the side view image.
Computing a plan view image of the analyzed data may include applying the following equation:
PlanViewmn=maxw|Imnw|2
where
maxw is the maximum value across all w (depths).
Computing a side view image of the analyzed data may include applying the following equation:
SideViewnw=maxm|Imnw|2
where
maxw is the maximum value across all w (depths).